In order to prevent counterfeiting, printing products such as bank notes and securities are often subjected to a process to impart a translucent property such as a “watermark” or a “security window” onto a sheet surface or to the inside of the sheet apart from ordinary pattern portions. The “watermark” is a distinguishable image or marking that appears more brightly on paper upon its irradiation with light (or more darkly with light reflected against a dark background). A typical watermark is textured in its manufacturing process by partially changing the thicknesses of the paper with a water-coated metal stamp or the like to form an embossed portion and a recessed portion on the paper. Another special watermark used in bank notes and the like is called a shaded watermark. In contrast to the typical watermark, the shaded watermark employs a technique to increase the thickness of a portion corresponding to an image or marking. In this way, the shaded watermark can express gradation of shades and thus achieve picturesque expression. Meanwhile, the “security window” means a transparent region consisting of a film of a transparent polymer sheet without base printing on both sides. A portrait or characters may further be printed in the security window.
Accordingly, printing products subjected to the process to impart a “watermark” or a “security window” need to be inspected for their presence and absence as well as their contours and locations.